# std::atomic<T>::fetch_add

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< cpp‎ | atomic‎ | atomic

member only of `atomic<Integral`﻿`>` specializations and `atomic<Floating`﻿`>` specializations(since C++20) |  |
T fetch_add( T arg, std::memory_order order = std::memory_order_seq_cst ) noexcept; | (1) | (since C++11)
T fetch_add( T arg, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; | (2) | (since C++11)
member only of `atomic<T*>` partial specialization |  |
T* fetch_add( std::ptrdiff_t arg, std::memory_order order = std::memory_order_seq_cst ) noexcept; | (3) | (since C++11)
T* fetch_add( std::ptrdiff_t arg, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; | (4) | (since C++11)

Atomically replaces the current value with the result of arithmetic addition of the value and arg. That is, it performs atomic post-increment. The operation is a read-modify-write operation. Memory is affected according to the value of order.

1,2) For signed integral types, arithmetic is defined to use two’s complement representation. There are no undefined results.

For floating-point types, the floating-point environment in effect may be different from the calling thread's floating-point environment. The operation need not conform to the corresponding std::numeric_limits traits but is encouraged to do so. If the result is not a representable value for its type, the result is unspecified but the operation otherwise has no undefined behavior. | (since C++20)

3,4) The result may be an undefined address, but the operation otherwise has no undefined behavior.
If `T` is not a complete object type, the program is ill-formed.

It is deprecated if std::atomic<T>::is_always_lock_free is false and overload (2) or (4) participates in overload resolution. | (since C++20)

### Parameters

arg | - | the other argument of arithmetic addition
order | - | memory order constraints to enforce

### Return value

The value immediately preceding the effects of this function in the modification order of *this.

### Example

Run this code

    #include <array>
    #include <atomic>
    #include <iostream>
    #include <thread>

    std::atomic<long long> data{10};
    std::array<long long, 5> return_values{};

    void do_work(int thread_num)
    {
        long long val = data.fetch_add(1, std::memory_order_relaxed);
        return_values[thread_num] = val;
    }

    int main()
    {
        {
            std::jthread th0{do_work, 0};
            std::jthread th1{do_work, 1};
            std::jthread th2{do_work, 2};
            std::jthread th3{do_work, 3};
            std::jthread th4{do_work, 4};
        }

        std::cout << "Result : " << data << '\n';

        for (long long val : return_values)
            std::cout << "Seen return value : " << val << std::endl;
    }

Possible output:

    Result : 15
    Seen return value : 11
    Seen return value : 10
    Seen return value : 14
    Seen return value : 12
    Seen return value : 13

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P0558R1 | C++11 | arithmetic permitted on pointers to (possibly cv-qualified) void or function | made ill-formed

### See also

atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) | adds a non-atomic value to an atomic object and obtains the previous value of the atomic (function template)
operator++operator++(int)operator--operator--(int) | increments or decrements the atomic value by one (public member function)

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